JP2657240B2 - Silicon casting equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting apparatus for silicon by electromagnetic induction and a continuous casting method using the same.

[Conventional technology]

As one of the continuous casting methods by electromagnetic induction, as shown in FIG. 4, a conductive bottomless crucible 6 divided into a plurality in the circumferential direction is installed in an induction coil 7 and a material 20 is placed therein.
There is known a method of solidifying by successively pulling downward while solidifying (hereinafter, continuous casting by electromagnetic induction means this method).

According to this method, since the bottomless crucible 6 is divided into a plurality in the circumferential direction, a current is generated in each crucible divided piece 16 by the current flowing through the induction coil 7, and the current is generated in the crucible 6.
An electric current is generated in the material 20 inside, and the material 20 is heated and melted. At the same time, a repulsive force is generated between the electric current flowing through the crucible segment 16 and the electric current flowing through the material 20, and the material 20
Can be maintained in a non-contact state.

However, in an actual operation, as shown in FIG. 5A, the molten material 20 in the crucible 6 is divided into crucible pieces.
It enters the gap 17 of 16 and solidifies. This phenomenon is called insertion, which makes it difficult to pull down the solidified material 20 from inside the crucible 6, and causes the solidified material inserted between the crucible pieces 16 to come into contact with the charged crucible surface to generate a discharge. For this reason, this method is practically used only in a form in which a slag is interposed between the crucible 6 and the material 20 in the crucible, and charging, melting, pulling down, and solidifying are continuously performed.

The continuous casting method by electromagnetic induction in which a slag is sandwiched between a crucible and a material in the crucible is called an induct slag melting method (Note), and is mainly used for melting and casting of an active metal such as titanium. Acts as a cushioning and insulating material between the crucible and the material in the crucible.

(Note) PGCLITES and RABEALL: Proc.the Fifth In
ternational Conf.on Electroslag and Special Meltin
g Technology (1975) P477 On the other hand, the directionally solidified ingot of silicon used as a material for solar cells, etc., conventionally dissolves silicon in a bottomed crucible under an atmosphere inert to molten silicon. Is solidified by pouring into a mold provided with a predetermined temperature gradient in the vertical direction and solidifying.

However, in this method, silicon contamination due to impurities from the crucible and the mold is inevitable. Further, in order to suppress this contamination, the crucible and the mold require particularly high-purity materials, and the cost of casting is significantly increased due to the complicated heating equipment for the mold. Further, since crystallization proceeds simultaneously from the bottom of the mold and the side of the mold, it is not preferable from the viewpoint of crystallography.

Therefore, introduction of the above-described continuous casting method by electromagnetic induction to silicon casting has been considered.

The continuous casting method by electromagnetic induction is a method that has been considered for a long time, but until the development of the indext slag melting method, power supply equipment and the balance with the power cost have hardly been implemented on an industrial scale. Was. However,
Recent significant technological advances have provided small, high-capacity power supplies and reduced power costs. Against this background, continuous casting by electromagnetic induction has attracted attention again as a silicon casting method.

As described above, the continuous casting method using electromagnetic induction has no material contact with the bottomless crucible, and completely prevents impurity contamination of silicon when used for casting silicon. If there is no contamination from the crucible, the material of the crucible can be reduced in grade, and the cost of equipment will be significantly reduced due to the absence of the need for a mold. Can be continuously manufactured at low cost. Further, it is very preferable because crystallization from the side wall of the crucible can be suppressed in crystallography.

Based on this idea, the United States filed a patent application in Japan for a continuous casting method of silicon by electromagnetic induction in Japanese Patent Application Laid-Open No. 61-52962.

[Problems to be solved by the invention]

However, the method disclosed in Japanese Patent Application Laid-Open No. Sho 61-52962 does not solve any problem of insertion which is a problem in continuous casting by electromagnetic induction.

Further, in this method, the temperature gradient when silicon is solidified and then cooled is large, and large thermal strain is generated and remains in the solidified silicon ingot, and cracks and crystal defects frequently occur in the silicon ingot. It was also found that satisfactory quality could not be obtained.

The insertion can be solved by supplying a large amount of power, but the quality defect due to thermal distortion cannot be solved at all by such a well-known technique, and becomes a fatal defect as a semiconductor material. In addition, if the insertion is suppressed by high power, the power cost is increased. Therefore, it is desired to suppress the insertion regardless of the high power if possible.

In view of such circumstances, it is a first object of the present invention to provide a continuous casting apparatus for silicon by electromagnetic induction which can completely prevent quality defects due to thermal distortion.

A second object of the present invention is to prevent the above-mentioned quality defects,
Another object of the present invention is to provide a continuous casting apparatus for silicon by electromagnetic induction and a casting method using the same, which can solve the problem associated with the insertion or insertion regardless of large power.

[Means for solving the problem]

The first apparatus of the present invention achieves the first object, and in a continuous casting apparatus for silicon by electromagnetic induction,
Immediately below the interface level between the molten silicon and the solidified silicon, heating means for the solidified silicon is provided along the pulling-down path of the solidified silicon.

The second to fourth devices of the present invention and the method of the present invention achieve the second object.

According to a second device of the present invention, in the first device, the circumferentially divided gap of the bottomless crucible is 0.3 to 1.0 mm.

According to a third device of the present invention, in the first device, the inner wall surface of the bottomless crucible is provided with a slope that extends outward at an angle of 0.4 to 2.0 ° downward.

According to a fourth device of the present invention, in the second device, the inner wall surface of the bottomless crucible is provided with a slope that extends outward at an angle of 0.4 to 2.0 ° downward.

In the method of the present invention, in the silicon casting method using any of the first to fourth apparatuses, the induction frequency is set to 0.5 to 200 KHz.
It is assumed that.

[Action]

According to the first to fourth devices and the method of the present invention, the cooling of the silicon ingot pulled down from the bottomless crucible is controlled by heating by the heating means, and a gentle temperature gradient is applied to the silicon ingot in the pulling down direction. Is obtained.

The operation of the second to fourth devices and methods of the present invention with respect to insertion is as follows.

The present inventors have determined that it is important to suppress insertion without supplying large electric power in order to industrially perform silicon casting by a continuous casting method using electromagnetic induction. Even if the insertion cannot be suppressed, if the ingot can be smoothly lowered, large power supply is not required. As a result of continuing research on specific measures from such a viewpoint,
From the former point of view, it was found that the management of the dividing gap in the circumferential direction of the bottomless crucible and the control of the induction frequency were effective, and from the latter point of view, the angle control of the inner wall surface of the crucible was found to be effective.

In other words, if the circumferential division gap of the bottomless crucible is 1.0 mm or less, the repulsion that occurs between the current flowing on the inner wall surface of the crucible and the current flowing on the surface of the molten silicon in the crucible is opposite to each other. The force works seamlessly in the split gap, effectively suppressing the insertion.

Fig. 3 shows the relationship between the induction frequency when silicon with diameters of 50mm and 120mm is continuously cast by electromagnetic power at a standard power (45 to 95kw) and the depth of insertion of molten silicon into the split gap. Is a graph in which is represented as a parameter.

When the division gap is 1.5 mm, the insertion depth exceeds 0.6 mm, but when the division gap is 1.0 mm or less, only the standard power mainly considering heat melting of silicon is given. Regardless, the insertion depth is 1/3 of 1.5mm
It is greatly suppressed to about 0.2 mm or less.

If the insertion depth is suppressed to 0.2 mm or less, there is a gap due to the repulsive force generated because the current flowing on the inner wall surface of the crucible and the current flowing on the skin portion of the molten silicon in the crucible are in the opposite direction. The lump can be lowered smoothly.

However, when the division gap is less than 0.3 mm, the current generated in the crucible divided piece rapidly reduces the efficiency of the current induced in the molten silicon in the crucible, causing unnecessary power consumption and economical melting. Can not be performed.

As described above, according to the second device of the present invention, the ingot can be smoothly lowered without supplying a large amount of power.

On the other hand, according to the investigation by the present inventors, the insertion becomes prominent below the induction coil 7 as shown in FIG. This is probably because the lower part of the molten silicon 13 in the crucible 6 receives a stronger vertical force due to its own weight.
Therefore, if the inner wall surface of the crucible 6 is provided with a slope that expands downward and outward, even if insertion occurs,
It can be lowered downward.

Table 1 shows the relationship between the inclination angle when silicon with a diameter of 50 mm is continuously cast with a standard electric power (45 to 55 kw) by crucible division gap 1.0 mm by electromagnetic induction and the judgment result of whether or not the ingot can be lowered. It is shown.

As is clear from the table, when the inclination angle is 0.4 ° or more, the ingot can be lowered.

However, if the inclination angle exceeds 2.0 °, the diameter below the crucible becomes excessively large.If the ingot pulling speed is increased, the molten silicon melts from the solid-liquid interface on the side of the ingot before the molten silicon solidifies. Silicon is dropped, and continuous ingot solidification cannot be performed.

From the above, according to the third device of the present invention, the ingot can be smoothly lowered.

Further, the fourth device of the present invention combines the management of the divided gap and the angle management of the inner wall surface, and makes it possible to further smoothly lower the silicon ingot from the bottomless crucible.

On the other hand, regarding the electrical conditions, the oscillation frequency at the time of induction heating is important in suppressing insertion and reducing power consumption. The induction frequency is a factor that determines the penetration depth of the current flowing through the inner wall surface of the crucible and the current flowing through the surface of the molten silicon in the crucible. The relationship between the induction frequency and the insertion depth is as shown in FIG. It is.

The lower the frequency, the greater the insertion depth, but under the condition that the dividing gap is controlled to 0.3 to 1.0 mm and / or the condition that the inclination angle of the inner wall of the crucible is controlled to 0.4 to 2.0 °, the induction frequency is 0.5. In the range of ~ 200KHz, the continuous lowering of the ingot is further smoothed and the power consumption is reduced.

The induction frequency of less than 0.5 kHz, the induction coils, overall frequency characteristics of a bottomless crucible and dissolved silicon not aligned, it is impossible to suppress the insertion of the dissolved silicon unless consume excessive power, even 200KH _z greater Similarly, the frequency characteristics between the induction coil, the bottomless crucible and the molten silicon deteriorate, causing unnecessary power consumption.

From the above, according to the method of the present invention, it is possible to lower the ingot more smoothly.

〔Example〕

FIG. 1 is a longitudinal sectional view showing the overall structure of one example of the first to fourth devices of the present invention.

Numeral 1 denotes an airtight container, which is a water cooling container having a double structure to protect the container from internal heat generation. Furthermore, it is connected to a vacuum exhaust pump via a duct so that the inside of the container can be evacuated, and also connected to an inert gas supply pipe so that the inside of the container can be controlled to an arbitrary pressure of the inert gas. .

The hermetic container 1 has a raw material charging container 4 and an ingot take-out chamber 5 separated by vacuum shut-off devices 2 and 3 at an upper portion and a lower portion, respectively. This can be performed while maintaining an inert atmosphere with respect to silicon.

A bottomless crucible 6 fixed substantially at the center of the airtight container 1
Has an induction coil 7 wound therearound and heating means
It is a structure in which 24 are connected. The structure of this part will be described later in detail with reference to FIG.

A raw material hopper 8 is provided below the raw material charging container 4 in the airtight container 1, and granular and massive raw material silicon 9 charged in the hopper 8 passes through a revolving charging duct 10 in a bottomless crucible. The molten silicon 13 is supplied.

A resistive heating element 11 made of graphite or the like is provided directly above the bottomless crucible 6 so as to be able to move up and down, and is inserted into the bottomless crucible 6 in a lowered state.

Below the heating means 24, a support and pull-out device 14 for pulling out the silicon ingot 12 while supporting it is provided.

The support and pull-out means 14 may be of a clamp roll type, but in consideration of the influence on the silicon ingot 12, the silicon ingot 12 is gripped without slip, and after descending a predetermined distance, the silicon ingot 12 is released. Then, it is preferable that a plurality of clamp bodies which are raised to the original position be simultaneously driven with a phase shift.

Further, for the silicon ingot 12, a cutting device using a laser beam or the like can be provided in the container 1.

FIG. 2 shows a structural example of a bottomless crucible and a heating means which are main parts of the first to fourth devices of the present invention.

The bottomless crucible 6 is a copper cylinder and is divided into a plurality of pieces in the circumferential direction except for the upper part. The inside of the crucible is a partition plate
The cooling water introduced into the crucible from the introduction pipe 18 descends on the inner part while circulating in the circumferential direction, cools the inner wall surface, and then rises on the outer part, and the outlet pipe 19 Is discharged out of the crucible 6 collected in the container.

The bottomless crucible 6 may have a gap of 0.3 to 1.0 mm between the crucible divided pieces 16 in the circumferentially divided portion, or may have an inner wall surface extending outward at an angle of 0.4 to 2 ° downward.
Alternatively, it has a structure satisfying both conditions.

In the illustrated example, the bottomless crucible 6 is divided in the circumferential direction while leaving the upper part, but may be left in the lower part or may be divided in the entire axial direction in the circumferential direction.

Further, the material is not limited to a cylindrical shape, and may be a rectangular tube shape, and the material may be a conductive material other than copper.

The induction coil 7 is provided concentrically around the bottomless crucible 6 and connected to a power supply by a coaxial cable (not shown). The induction coil 7 and the coaxial cable are cooled using the same cooling water channel.

The heating means 24 is provided concentrically below the bottomless crucible 6 and has a metallic resistance heating element 22 outside a cylindrical shielding plate 21 made of a quartz tube, and the upper part is denser than the lower part. It has a structure in which it is wound and its outside is further covered with a heat retaining furnace wall 23 mainly composed of alumina.

The heating element 22 may be divided into a plurality of blocks in the vertical direction so that the heating can be controlled independently.

6 (a) and 6 (b) show another example of the heating means 24. FIG.

The heating means 24 shown in FIG.
And a second induction coil 31 provided on the outer side of the cylindrical conductor 30 concentrically connected immediately below the conductor 30.
Is covered with a heat insulating wall 32. Second induction coil
31 may use a common power supply with the induction coil 7 provided in the bottomless crucible 6 or an independent power supply. As the material of the conductor 30, for example, graphite, Ta, Mo, or the like is selected, and as the material of the heat retaining wall 32, for example, a graphite fiber molded body, alumina, or the like is selected.

The heating means 24 shown in FIG.
Is extended downward to form a heating means 24. Second induction coil 31 for heating solidified silicon provided around the extension
Similarly to the example shown in FIG. 6 (a), a power supply common to the induction coil 7 or an independent power supply may be used. When the heating coil 7 and a common power supply are used, it is necessary that the second induction coil 31 can be independently controlled with respect to the induction coil.
When any of the heating means 24 is used, a desired axial temperature gradient is applied to the silicon immediately after solidification, and the occurrence of thermal distortion due to rapid cooling is prevented.

The specific procedure of the silicon casting method using the apparatus shown in FIGS. 1 and 2 will be described below.

First, after the inside of the airtight container 1 is evacuated, argon is sealed as an inert gas. Then, the cylindrical seed ingot made of silicon is supported and pulled out by the device 14 so that the upper end of the seed ingot is a bottomless crucible 6. So that it is located at the center in the height direction.

Next, with the charging duct 10 above the bottomless crucible 6 retracted in the horizontal direction, the heating element 11 made of graphite or the like is used.
Is lowered and inserted into the bottomless crucible 6 so as to be set immediately above the seed ingot, and energization of the induction coil 7 is started.

When the upper end surface of the seed ingot in the induction coil 7 is melted, the heating element 11 is raised and returned to the original position, and when the seed ingot is raised in the crucible 6, molten silicon 13 is initially formed.

Immediately after the initial formation of the molten silicon 13, the granular raw silicon 9 is added to the molten surface of the molten silicon 13 from the charging duct 10 to melt the raw silicon 9, and the supporting and By operating the drawing device 14, the silicon ingot 12 including the seed ingot
From the bottomless crucible 6 and the heating means 24.

As a result, the melted silicon 13 is sequentially separated from the region where the electromagnetic force acts, and solidifies continuously from the portion in contact with the silicon 12, and the portion just before the solidification is heated in the desired axial direction by the heating means 24. A temperature gradient is applied.

By continuing the above operations of continuous raw material charging, melting, and solidification, a high-quality directional solidified ingot of silicon without thermal distortion can be manufactured.

The manufactured silicon ingot 12 is moved to the discharge chamber 5 with the shut-off device 3 released, and is extracted outside the discharge chamber 5 after the shut-off device 3 is closed.

The raw material silicon 9 is supplied from the charging container 4 into the hopper 8 by releasing the shut-off device 2 and then is supplied in a predetermined amount.
The shutoff device 2 is closed again to prepare for the next casting.

The results of actual casting according to the above procedure are described below, including comparative examples.

Induction frequency using two band 0.5~20KH _z, 100~200KH _z.

For 100～200KH _z, inside diameter 50 mm, circumferential division number at the outer diameter 80mm is 12, split gap 1.5,1.0,0.3Mm, copper bottomless crucible shape shown in Figure 1 crucible divided piece height 150mm It was used. The inclination angle of the crucible inner wall surface is basically 1.0 °, and 1.0 ° for the one with a division gap of 1.0 mm, and 0.1 °.
There are five types: 2, 0.4, 2.0, and 5.0 mm.

The induction coil combined with this bottomless crucible has an outer diameter of 90 mm,
It is a water-cooled copper-cooled 4-turn 45mm high. The electric power was 45-55kw, mainly considering the heating and melting of silicon.

For 0.5～20KH _z, internal diameter 120 mm, circumferential division number at the outer diameter 150mm is 24, split gap 1.5,1.0,0.3Mm, height 150mm crucible divided pieces, the tilt angle 1.0 ° in crucible wall A bottomless crucible having the shape shown in FIG. 1 was used.

The induction coil combined with this bottomless crucible has an outer diameter of 160m
Power of 70 to 95 kw was supplied by a water-cooled 4-turn copper-made 45 m high, 45 mm high.

The ingot lowering speed was 1.5 mm / min in each case.

The heating means 24 has the structure shown in FIG. 2 and has a heating effective portion of 60 mm in inner diameter (for a bottomless crucible having an inner diameter of 50 mm) or 130 mm (for a bottomless crucible having an inner diameter of 120 mm) and a height of 15 mm.
Each of the cylinders having a diameter of 0 mm was configured such that the temperature at the upper end of the bottomless crucible side was 1050 ° C. and the outlet temperature at the lower end was 650 ° C.

FIG. 3 and Table 1 described above summarize the results of casting under the above conditions.

In addition, about what was possible to lower the ingot,
When a circular substrate with a diameter of 50 mm and a thickness of 0.35 mm was cut out from the ingot with a diamond cutter, the circular substrate did not break during cutting, indicating that no thermal distortion remained in the ingot. Was. Further, when the diffusion length of the semiconductor circular substrate was measured, the semiconductor P-type 1
It showed 200 to 300 μm in Ωcm, and it was also found that the semiconductor polycrystal produced by the apparatus and method of the present invention can be sufficiently used as a solar cell substrate.

〔The invention's effect〕

The first to fourth apparatuses and methods of the present invention are capable of producing high-quality, low-cost silicon by electromagnetic induction while continuously casting silicon by electromagnetic induction without causing critical heat distortion in the ingot as a semiconductor. Enable casting.

In addition, the second device of the present invention regulates the divided gap of the bottomless crucible so as to suppress the insertion of silicon into the divided gap to such an extent that there is no support for lowering without supplying a large power.

The third device of the present invention allows the inner wall surface of the bottomless crucible to be inclined smoothly by lowering it even if silicon is inserted.

The fourth device of the present invention further smoothes down the silicon ingot by regulating the split gap and regulating the angle of the inner wall surface.

As a result, any of the devices enables economical and efficient continuous silicon casting on an industrial scale by electromagnetic induction, and exhibits a great effect in reducing the cost of casting silicon.

In addition, the method of the present invention further facilitates lowering of the silicon ingot by combining frequency management with the first to fourth devices.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view illustrating the entire structure of the device of the present invention,
FIG. 2 is a perspective view showing an example of the structure of the bottomless crucible and the heating means, which are the main parts of the apparatus of the present invention. FIG. 3 quantitatively shows the effects of the divided gap and the induction frequency of the bottomless crucible on the insertion. Graphs, FIGS. 4 (a) and 4 (b) are plan and longitudinal sectional views showing the basic principle of continuous casting by electromagnetic induction, and FIGS. 5 (a) and 5 (b) are transverse sectional views showing the insertion phenomenon. And longitudinal sectional views, FIG. 6 (a) and (b)
FIG. 7 is a longitudinal sectional view showing another example of the structure of the heating means. In the figure, 6: bottomless crucible, 7: induction coil, 24: heating means.

Claims (5)

(57) [Claims] A conductive bottomless crucible, at least part of which is divided into a plurality in the circumferential direction, is installed in an induction coil, and silicon is brought into contact with the crucible inner wall by the bottomless crucible. In a continuous casting apparatus for silicon by electromagnetic induction, which is melted by electromagnetic induction and pulled down and solidified, a heating means for solidified silicon is disposed along a lowering path of solidified silicon immediately below the interface level between molten silicon and solidified silicon. A silicon casting apparatus characterized by the above-mentioned. 2. The silicon casting apparatus according to claim 1, wherein a circumferential divided gap of said bottomless crucible is 0.3 to 1.0 mm. 3. The silicon casting apparatus according to claim 1, wherein the inner wall surface of said bottomless crucible has a thickness of 0.4 to 2.0.
A silicon casting apparatus characterized by having a slope extending outward at an angle of ゜. 4. The silicon casting apparatus according to claim 2, wherein the inner wall surface of the bottomless crucible has a thickness of 0.4 to 2.0
A silicon casting apparatus characterized by having a slope extending outward at an angle of ゜. 5. A silicon casting method using the silicon casting apparatus according to claim 1, wherein the induction frequency is 0.5 to 200 KHz.